The present invention relates to a signal switching method and apparatus, and more particularly to a signal switching method and apparatus capable of executing a protection switching to a signal closer to normality by dividing signals to be switched at a transmittal terminal and comparing the divided signals with each other at a reception terminal.
Conventionally, such a type of art was proposed as one technique for restoring a failure in case that topology of a network is a ring in a STM (Synchronous Transfer Mode) system such as a SONET (Synchronous Optical Network) (SONET PPS Self-healing Ring (TA-NWT-001400 Issued on Apr. 1, 1993, Bellcore), SONET Dual-Fed Unidirectional Path Switched Ring (UPSR) Equipment Generic Criteria (GR-1400-CORE Issued on Mar. 1, 1994, Bellcore)). Also, in this technique, the relation between bands in current use and spare use was that the spares always have one band whenever there was one band for the current use. In other words, the failure was restored by making entirely the same signals exist for the current use and the spares concurrently and switching the signal in the current use to one in the spares on a reception side when some failure occurred in the current use.
FIG. 10 shows the conception of the prior art. A case is considered here that a signal 2-1 to be input from a node 1 is transmitted to a node 3. In this case, the signal 2-1 to be transmitted diverges (is bridged) at the node 1 and the divergence signals are added and sent to rings 2-2 and 2-3 in opposite directions. At a node 3, in turn, signals 2-4 and 2-5 are dropped from the signals flowing through the rings 2-2 and 2-3, which are transmitted via different routes, respectively, and the dropped signals are compared with each other at a selector (a two to one selector) 2-6 to select a signal closer to normality as selected signal 2-7, resulting in restoring a failure.
As a concrete method for implementing the prior art, there was a method of collectively dealing with entire transmittal data by one switch in order to select a correct signal by the Pass-Protection-Switch (PPS). FIG. 11 illustrates this method. In FIG. 11, the same signals 3-8 and 3-10 transmitted through different routes, respectively, from signals OC-3 (Optical Carrier-Level 3) flowing through two rings are dropped into network interface packages 3-1 and 3-2, and in order to compare the two signals 3-8 and 3-10 with each other, the signals 3-8 and 3-10 are input to a concentrated switch portion 3-4 of a switch package 3-3 from the network interface packages 3-1 and 3-2. A monitor portion (MON) 3-5 monitors normality of the two signals 3-8 and 3-10 input to the concentrated switch portion 3-4 and supplies a monitoring result to the concentrated switch portion 3-4. Under control of a CPU 3-6, the concentrated switch portion 3-4 collectively switches and controls the signals 3-8 and 3-10 and provides a user interface package 3-7 with a signal having high normality out of the two signals 3-8 and 3-10. In addition, reference numerals denote signals that are output from a subordinate portion of the concentrated switch portion 3-4 and added to the signals flowing through the two rings.
Also, as another method, as shown in FIG. 12, there was a method of executing a switching by dropping the same signals 3-8 and 3-10 transmitted through different routes, respectively, from signals OC-3 (Optical Carrier-Level 3) flowing through two rings into user interface package 4-3, and selecting a signal closer to normality out of the signals 3-8 and 3-10 in a switch portion 4-6 included in the user interface package 4-3 as a selected signal (JP-A-107425/1996 shall be referred to). This user interface package 4-3 further has a monitor section (MON) 4-7 for monitoring normality of the signals 3-8 and 3-10, an OAM signal generating section 4-8 for generating an OAM (Operation, Administration and Maintenance) signal to a subordinate device (user interface) side, an OAM inserting portion (MUX) 4-9 for inserting the OAM signal into the selected signal, a selection portion 4-4 for selecting whether a signal from a West side passes through and goes to an East side or a signal from the subordinate device side is added to a signal from the West side, and a selection portion 4-5 for selecting whether a signal from the East side passes through and goes to the West side or a signal from the subordinate device side is added to a signal from the East side.
Any method of these is a technique wherein, as mentioned above, a network of the STM representative of the SONET is formed of rings and one signal is selected from two signals in the current use and the spares.
In JP-A-132920/1994, a switching circuit for a transmittal device having redundant construction is proposed for the purpose of, even though a failure occurs in a unit switching device, preventing signals from both of units from colliding with each other or preventing a signal from not being output from any unit, and the switching circuit has means for outputting a signal indicative of operating condition of its unit to each of other units and means for, to the contrary, inputting a signal indicative of operating condition of other units, and is for executing a switching of units for current use and spare use based on these signals indicative of the operating condition.
In JP-A-30571/1995, an interlocking device is proposed for the purpose of, in a self-healing ring, making the extension of a network thereof, wherein the whole network can be made to a self-healing system by applying a route diversity mechanism to parts interlocked to a ring network.
In JP-A-69944/1994, a drop/insert multiplexing tributary station device for a ring-shaped transmittal line is proposed, wherein in case that a failure occurs in the ring-shaped transmittal line, reliability of each channel is improved by enabling a digital signal to go round transmittal in an opposite direction per channel unit in which the digital signal is converted into a voice-band signal.
In JP-A-154409/1995, an AIS (Alarm Indication Signal) insertion and removal system for inserting the AIS into a accident relief ring N/W and a drop path at each node thereof is proposed, which is implemented by positively dealing with nothing particular when detecting the AIS on an upstream side of a signal for the purpose of shortening an AIS insertion time to a downstream side.
As explained above, the prior arts are proposed as a method or system for restoring a failure in case that a topology of a network is a ring in an STM system such as a SONET, and thus, a fine switching per ATM (Asynchronous Transfer Mode) layer, for example, as the unit of a VP (Virtual Path) and a VC (Virtual Channel) cannot be executed in the prior arts, and since a topology other than a ring topology is not taken into account, the prior arts cannot be applied to a mesh topology.
Also, in this method or system, since the relation between bands in current use and spare use was that the spares always have one band whenever there was one band in the current use, there is a problem that adaptation to a wide switching method such as protection for m (m is integers equal to or more than two) spares to n (n is integers equal to or more than two) of the current use has not been taken into account.
Also, as a concrete implementation method, conventionally, in order to select a correct signal by means of a path protection switch (referred to as PPS, hereinafter), whole transmittal data were input to one switch and switched by a CPU processing collectively. In this method, there are problems that in case that a lot of control commands of a switching occur concurrently, a bottleneck of the CPU processing spends much time and that reliability becomes to be extremely lowered due to the necessity of a switch having a complicated circuit.
Also, in the aforementioned JP-A-107425/1996, a switching method was proposed in which the switching is executed by a switch for selecting one signal of two signals. However, there is room for improving the method in order to implement it at a low price and compactly.